PORTLAND - Invading plant species, an ecological problem once relegated to the back burner, have surged to the forefront of issues dominating environmental conversations, one expert says, and a hugely successful 25-year fight against one weed in the Pacific Northwest may hold many valuable lessons for how to deal with an increasing onslaught of other invasive species.

The war against tansy ragwort in Oregon has essentially been won, says Peter McEvoy, a professor of ecology at Oregon State University, who spoke today at the annual meeting of the Ecological Society of America.

The plant now exists at 1-3 percent of its former abundance. The diversity of other plants in places once infested by tansy has greatly increased, and some native plant species that were on the brink of localized extinction have made recoveries.

That's important in itself, said McEvoy, who has spent much of his career studying natural, "biological" controls to use against this noxious weed, which once cost Oregon farmers and ranchers $5 million a year in health impacts on their livestock that ate the pretty but poisonous yellow weed.

But the real value of the fight against tansy could be in the lessons it provides botanists and land managers to use in their war against other plant invasions, McEvoy said.

"The biggest part of our fight against tansy was that our view of biological control was too simplistic," McEvoy said. "In the old days, the idea was simply that invading species were a problem because they lacked natural enemies in their new environment. Adding those enemies would be the cure."

But that rationale, he said, failed to explain why only about one new species in 10 becomes invasive, or why only one in five introduced natural enemies is effective in controlling the target weed. A complex understanding of a species' ecology could be used against it to form an attack on multiple fronts that would be far more effective than any one approach.

The fight against tansy began with importation of the cinnabar moth, an insect species whose caterpillar had an affinity for eating tansy leaves. It had some success. But before the battle was over, researchers had also added plant competition concepts and the ragwort flea beetle to the mix. They studied 24 different treatment combinations, and used a complex manipulation of disturbance, competition and herbivore regimes to achieve optimal results while minimizing the need for repeated and costly interventions.

"We've learned a lot about how to attack invading plant species by learning how they become invasive and how we can interrupt that cycle," McEvoy said. "There may be ways we can reduce their movement, import their natural enemies, provide more competition for them, or use other tools to win the fight."

And the next major step forward, he said, may be with improved tools to predict and prevent plant invasions in the first place.

"Looking back, we see now how we could have predicted that tansy would take off and become such a huge problem," McEvoy said. "Its generation time is very short; its seed production is prolific. There are things we can learn here."

Part of the problem, he said, is that new plant species are constantly being imported into new places, often on purpose to fit the needs of the ever-changing horticultural industry. Some of the deliberate introductions include salt cedar, which is now raging through the Southwest, drawing down water tables and leaving a layer of salt behind. Purple loosestrife is destroying the plant diversity of Oregon wetlands and ruining wildlife habitat. Japanese knotweed is taking over riparian zones. False brome, a grass, is displacing native plants in Oregon grasslands and forests.

And each of these plants were deliberately imported, McEvoy said, adding to an invading plant species problem that one study estimated to cost $34 billion a year in the U.S. - or $137 billion a year if you add in the crises with all invading species, including animals.

In Oregon alone there are 70 control organisms being used on 30 invading weed species. The problem is sufficiently severe, McEvoy said, that in the future it may be necessary to fight more than one invading species at a time in many locations.

The safety of biological controls and their increasingly widespread use is also drawing more scrutiny, he said.

"As we bring in more pathogens or animals to use as biological controls, we have to develop better ways of learning what risks they may pose in themselves," McEvoy said. "For instance, gall flies were imported to help control knapweed. But no one anticipated that deer mice would feed on the gall flies, and the population of these mice doubled. The mice can carry hantavirus, so now you're talking about an increased human health threat."

The key to biological controls is to understand these complexities and use them to our advantage, not to create new problems, McEvoy said.

"The potential of ecology to predict, prevent or control invading plants is huge," McEvoy said. "The science is becoming more powerful all the time. But the problems are also getting worse, and the stakes are high."